Responding to recent requirements stricter quality control of steel products, a good deal of effort has been made to reduce non-metallic inclusions, such as alumina which builds up on an inner hole portion of a nozzle for continuous casting of steel. Moreover, in terms of casting operations, the buildup of alumina and others which arises from long hours of casting is likely to cause clogging of the inner hole portion and difficulties in continuing the casting operation, which hinders the improvement in productivity.
As one of measures against the alumina buildup, there has been known a technique in which argon gas is injected from the inner surface of a nozzle into molten steel to prevent the alumina buildup in physical manner. In this technique, if the argon gas is injected at an excessive amount, bubbles of the injected argon gas will be incorporated into molten steel to form pinholes in slabs. Thus, due to the restriction in the allowable injection amount of argon gas, this method cannot be exactly used as a sufficient measure for preventing the alumina buildup.
There has also been known a technique intended to provide an anti-alumina-buildup function to refractories themselves. For example, in view of inducing the reaction between alumina attached on a refractory nozzle and CaO contained in the refractories to form a low-melting-point or fusible substance, Japanese Patent Publication No. 61-44836 discloses a casting nozzle using refractories containing a combination of graphite, and either one of sintered calcia, fused calcia and another ceramic material containing a CaO component. While such CaO-containing refractories can exhibit an anti-alumina-buildup effect in some cases, it is likely to cause increase in alumina buildup depending on the circumstances.
Further, when applying refractories containing CaO as a mineral phase to a casting nozzle, the CaO is likely to undesirably hydrated due to a hydration reaction with water absorbed therein. While Japanese Patent Laid-Open Publication No. 10-5944 discloses a technique of adding CaCO3 as aggregate to prevent the hydration, the CaCO3 aggregate is pyrolytically decomposed to have a high porosity, and the resulting irregularity of an effective or operative surface of a casting nozzle leads to undesirable alumina buildup thereon.